Inter-Cell Interference (ICI)
A wireless cellular network includes base stations (BSs) and mobile stations (MSs) arranged in cells. The BSs and MSs include transceivers. The mobile stations are frequently called ‘users’ or subscriber stations. The stations communicate signals with each other using channel resources, i.e., time and frequency. The same resources may be used in adjacent cells. This can cause inter-cell interference (ICI) in regions where the signals in the cells physically overlap. Because the power of the ICI at a receiver can be much greater than the power of noise, ICI is a major limitation of the spectral efficiency of wireless cellular networks.
Spatial Division Multiple Access (SDMA)
SDMA provides multi-user channel access by using multiple-input multiple-output (MIMO) techniques with precoding and multi-user scheduling. SDMA exploits spatial information of the location of MSs within the cell. With SDMA, the radiation patterns of the signals are adapted to obtain a highest gain in a particular direction. This is often called beam forming or beam steering. BSs that support SDMA transmit signals to multiple users concurrently using the same resources. SDMA can increase network capacity, because SDMA enables spatial multiplexing. Nevertheless, the ICI still remains a key issue, even if SDMA is used.
Macro Diversity Handover (MDHO)
During macro diversity handover, multiple base stations transmit the same signal to single MS in a handover (HO) region. Macro diversity increases the received signal strength and decreases fading in the HO region. MDHO is used when the MS moves from one cell to another. The transfer is accomplished in the downlinks (DLs) from the BSs to the MS by having the BSs transmit multiple copies of the same information to the MS so that either RF combining or diversity combining can be performed by the MS.
In the uplink (UL) from the MS to the BSs, the transfer is accomplished by having two or more BSs receiving the same signal from the MS in the HO region so that selection diversity can use the ‘best’ uplink. MDHO can reduce the ICI even though the same resources are used for the same signal. However, MDHO wastes resources because the MS uses the resources from more than one cell, which could otherwise be used by other MSs.
Diversity Set Establishment
As shown in FIG. 3, conventional MDHO begins when a MS is detected 305 in the HO region. Either the MS or the BS can initiate establishment of a diversity set 310, as determined in step 311. The diversity set 310 consists of only the base stations that are capable of communicating with the MS in the HO region. In the case of the BS, the BS transmits a MOB_BSHO_REQ message 312. In the case of the MS, the MS transmits a MOB_MSHO_REQ message 313 and a MOB_BSHO_RSP message 314. In either case, the diversity set is confirmed with the MOB_HO_IND message 315. The messages are defined in detail in the applicable standards.
According to the IEEE 802.16 standard, MDHO and SDMA are optional and independent protocols between the BSs and the MSs. It is desired to improve cooperation between stations in wireless cellular mobile wireless networks using MDHO and SDMA.